1. Technical Field
The present invention relates to a flow rate measurement device for measuring a flow rate of a gas, for example to a flow rate measurement device used in medical instruments such as treatment equipment for sleep apnea syndrome, industrial instruments for monitoring an air flow rate in factories, use in instrument embedment, and the like.
2. Background Art
In general, in a straight pipe type flow rate measurement device for directly measuring a great flow rate among flow rate measurement devices, there is a need for enlarging a diameter of a pipe in order to lower flow speed by a measurable area of a flow rate detection element. Thus, downsizing of the device is limited. In order to downsize the flow rate measurement device for measuring a great flow rate, in general, a flow rate measurement device having a bypass flow passage structure is proposed. In the flow rate measurement device having the bypass flow passage structure, an auxiliary flow passage diverges from a main flow passage, flow speed of a gas flowing in the auxiliary flow passage is measured by a flow rate detection element, and a total flow rate is determined by a diversion ratio of the main flow passage and the auxiliary flow passage and the flow speed in the auxiliary flow passage.
However, in such a flow rate measurement device having the bypass flow passage structure, flow speed distribution on a vertical section in the main flow passage easily becomes inhomogeneous due to existence of the auxiliary flow passage. When the flow speed distribution in the main flow passage is inhomogeneous, the diversion ratio to the auxiliary flow passage becomes unstable. Thus, a precise total flow rate is not easily determined.
(Patent Document 1)
A flow rate measurement device having a bypass flow passage structure for improving a decrease in measurement precision due to such inhomogeneity of the flow speed distribution is shown in FIG. 1 (refer to Patent Document 1). In this flow rate measurement device 11, an orifice (not shown) is provided in a main flow passage 13, and an auxiliary flow passage 14 is provided on a main flow pipe 12 astride an outer peripheral surface of the main flow pipe 12 having the main flow passage 13.
The auxiliary flow passage 14 has the following structure. On an inner wall of the main flow passage 13, introduction ports 15 are provided on both the left and right sides on the upstream side of the orifice, and discharge ports 16 are provided on both the left and right sides on the downstream side of the orifice. Upper ends of introduction flow passages 17 (introduction vertical grooves) extending upward from the introduction ports 15 and upper ends of discharge flow passages 18 (discharge vertical grooves) extending upward from the discharge ports 16 are connected to each other by first auxiliary flow passages 19 so as to communicate with each other. The upper ends of the left and right introduction flow passages 17 are connected to each other by a second auxiliary flow passage 20 so as to communicate with each other, the upper ends of the left and right discharge flow passages 18 are connected to each other by another second auxiliary flow passage 21 so as to communicate with each other, and center parts of both the second auxiliary flow passages 20, 21 are connected to each other by a horizontal detection flow passage 22 so as to communicate with each other. A flow rate detection element (not shown) for measuring flow speed of a gas is provided in the detection flow passage 22.
When the gas passes through the interior of the main flow passage 13 of this flow rate measurement device 11, part of the gas flows from the introduction ports 15 into the auxiliary flow passage 14, returns from the discharge ports 16 to the main flow passage 13 through a route as shown by arrows in FIG. 1, and then flows in the main flow passage 13 again. At this time, the gas flowing into the introduction flow passages 17 bifurcates and flows into the first auxiliary flow passages 19 and the second auxiliary flow passage 20, so that part of the gas flows into the second auxiliary flow passage 20. The gas flowing into the second auxiliary flow passage 20 from the left and right sides together flows into the detection flow passage 22. The flow speed of the gas passing through the detection flow passage 22 is measured, and a total flow rate is determined by a diversion ratio of the main flow passage 13 and the detection flow passage 22, and the flow speed in the detection flow passage 22.
(Patent Document 2)
Similarly, in a flow rate measurement device disclosed in Patent Document 2, an orifice is arranged in a main flow passage, four introduction flow passages and four discharge flow passages forming an auxiliary flow passage diverge from the main flow passage respectively on the upstream side and the downstream side of the orifice, a gas introduced from the introduction flow passages is collected into one detection flow passage, deviation of flow speed distribution is averaged, and then flow speed of the gas is measured by a flow rate detection element.
Patent Document 1: Japanese Patent No. 3870969 (FIG. 15)
Patent Document 2: U.S. Pat. No. 5,279,155